This invention relates to a circuit arrangement for operating a high-pressure sodium discharge lamp provided with two lamp connection points for connection of the highpressure sodium discharge lamp and comprising a controlled main switching element, a control electrode of which is connected to a control circuit, a measuring impedance connected in series with a lamp connection point and with a measuring impedance in parallel with the lamp connection points, which measuring impedances are further connected to the control circuit.
A circuit arrangement of the kind mentioned in the opening paragraph is known from European patent application EU 0 228 123 (PHN 11.705). In the known circuit arrangement, the measuring impedances are in the form of resistors and the circuit arrangement is constructed so that in the operating condition a signal S is generated across the resistors, which signal is a summation of a signal part proportional to the voltage across the lamp (lamp voltage) and a signal part proportional to the current through the lamp (lamp current). It is further ensured that the polarity of the generated signal S corresponds to the polarity of the signal part proportional to the lamp current.
It is common practice that high-pressure discharge lamps are operated at alternating voltage or at a pulsatory direct voltage. The power with which the lamp is operated is to be understood herein to mean the power averaged over a time which is long as compared with the period of the lowest occurring frequency of the voltage at which the lamp is operated. An average lamp voltage or current is to be understood to mean a voltage or current formed by averaging in time the absolute value of the lamp voltage or lamp current. Another way in which an averaged lamp voltage or lamp current may be formed consists in using the root of the value averaged in time of the square value of the lamp voltage or current, the so-called RMS value. The actual lamp voltage will have periodically a comparatively short time duration with a peak-shaped voltage, the so-called recognition peak, followed by a time duration with a comparatively high and approximately constant value. The comparatively high approximately constant value is known as the "plateau voltage".
Nominal lamp current and lamp voltage, respectively, are the nominal values of the averaged lamp current and lamp voltage, respectively.
This known circuit arrangement makes it possible to operate by means of a control process effected in the control circuit, a high-pressure sodium lamp approximately at a constant averaged lamp voltage, a comparatively short time constant of the control process being sufficient withstanding the fact that high-pressure sodium discharge lamps have the property that upon an abrupt variation of the averaged lamp current, the averaged lamp voltage varies abruptly with an opposite polarity and then varies gradually with the same polarity as the current variation until a stable work-point associated with the varied lamp current is attained.
However, in the known circuit arrangement, a control process with a short time constant is only possible due to the fact that the signal S applied to the control circuit comprises a constant fraction of the lamp current. This results in only approximately a lamp operation with constant lamp voltage. This has the disadvantage that a quantity very strongly dependent upon the averaged lamp voltage, such as the colour temperature T.sub.c of the light emitted by the lamp, remains constant only to a rough approximation.
Another aspect of the known circuit arrangement is that, in order to obtain a closest possible approximation of a lamp voltage control, the part of the signal S proportional to the lamp current is chosen to be just so large that the polarity of the signal S is equal to that of the part proportional to the lamp current also immediately after the occurrence of an abrupt variation of the lamp current and lamp voltage. This has the consequence that the initial value of the signal S is very limited, irrespective of the value of each of the proportional parts. This leads to a certain inertia of the control process and hence to a limitation in the approximation of a constant averaged lamp voltage and constant colour temperature T.sub.c.